Hornblende chemistry in Southern Appalachian granitoids : implications for aluminum-hornblende barometry and Alleghanian tectonics

Recent empirical and experimental calibration studies have demonstrated that the total aluminum content of hornblende in equilibrium with quartz, two feldspars, biotite, titanite, and iron-titanium oxide is a function of pressure in calc-alkaline rocks. Previous authors have proposed that the pressure-sensitive hornblende substitution . IV VI IV IS the tschermakite substitution (Si^IV + Mg^VI = Al^IV + Al^VI) . To test this hypothesis, hornblendes in 15 late Paleozoic granitoids from the southern Appalachian Piedmont have been analyzed by electron microprobe. All of these plutons contain the above assemblage, and equilibrium between hornblende, plagioclase and biotite is confirmed by constant distribution coefficients. Textural and chemical relationships between these phases are consistent with equilibrium crystallization according to the reaction: liquid 1 + amphibole + ilmenite = biotite + quartz + anorthite + orthoclase + titanite + magnetite + liquid 2, where interstitial melt represents both the source of liquid 1, and the sink for liquid 2. Observed chemical variation in the Piedmont granitic hornblendes is attributed to hastingsite, pargasite, and "CANT" (Ca^M4 + Al^VI = Na^M4 + Ti^VI) substitutions. (The tschermakite component does not occur independently of edenite, as previously proposed.) Application of aluminum-hornblende barometry to these IV granitoids indicates that the hastingsite component in hornblende increases linearly with increasing pressure, and that a substitution of the form [ ]^A + Mg^VI + 2Si^IV = Na^A + (Fe⁺³, Al) + 2Al may be the common pressure-sensitive aluminum substitution in natural hornblendes from calcalkaline plutons. In addition, aluminum-hornblende barometry of these granitoids quantifies previously unrecognized differences in depths of pluton emplacement that are systemmatically related to lithologic/structural belts of the southern Appalachian Piedmont: Plutons were emplaced in the Charlotte belt at depths of 11.6-17.3 km and in the Carolina slate belt at depths of 10.8-11.6 km, and are currently exposed at the same erosional level as a result of Alleghanian folding. Alleghanian peak metamorphic pressures in the Kiokee-Raleigh belt and aluminum-hornblende pluton emplacement pressures in the adjacent slate belts indicate that the Kiokee-Raleigh belt represents a "ductile horst" which experienced up to 15.8 km of vertical uplift during the Alleghanian event.